The present invention relates to a ball screw and, in particular, to an improvement in lubricant supply means which supplies lubricant such as grease into between a ball screw shaft and a ball nut.
A ball screw for use in table feed apparatus is conventionally structured in such a manner as shown in FIG. 6. In FIG. 6, reference character 11 designates a ball screw shaft, 12 designates a ball nut, and 13 designates a plurality of balls serving as rolling elements. And, in the outer peripheral surface of the ball screw shaft 11, there are formed first spiral groove 14. On the other hand, in the inner peripheral surface of the ball nut 12, there are formed second spiral groove 15; and, the balls 13 are allowed to roll along rolling passages respectively formed between the second and first spiral grooves 15 and 14.
Also, in the interior portion of the ball nut 12, there are formed rolling body circulation passages (not shown). The balls 13 are allowed to roll repeatedly along the rolling passages which are formed between the first and second spiral grooves 14 and 15, and the rolling body circulation passages. Further, a pulley 16 is mounted on the ball nut 12, and, in case where the pulley 16 is rotated by a motor 19 through a belt 17 and another pulley 18, the balls 13 are allowed to roll repeatedly between the first and second spiral grooves 14 and 15. With the repeated rolling movements of the balls 13, the ball screw shaft 11 is moved in the axial direction thereof with respect to the ball nut 12. By the way, in FIG. 6, reference character 20 designates a seal member for sealing a clearance between the ball screw shaft 11 and ball nut 12.
In the above structured ball screw, in case where the ball screw shaft 11 and ball nut 12 are worn due to the rolling movements of the balls 13, since the feed accuracy of the ball screw is degraded, a grease nipple 21 is mounted on the ball nut 12. That is, lubricant such as grease is supplied from the grease nipple 21 into the interior portion of the ball nut 12 to thereby control the wear of the ball screw shaft 11 and ball nut 12. However, when the ball screw is used under a high load, an oil film is easy to break and thus the lubricant must be supplied frequently into the interior portion of the ball nut 12 in order to prevent such oil film breakage, which results in a large amount of lubricant used.
Also, conventionally, when filling grease into the ball nut 12 from the grease nipple 21, because the viscosity of the grease is high, the grease oozes out from the two ends of the ball nut 12 before the grease is filled in sufficiently. Further, even the grease filled into the interior portion of the ball nut 12 is also pushed out to the outside of the ball nut 12 due to the operation of the ball nut 12. Therefore, the peripheral portion of the ball nut 12 is soiled with the grease, which requires an extra amount of grease.
As described above, in the conventional ball screw, since the lubricant such as grease supplied into the interior portion of the ball nut 12 is pushed out to the outside of the ball nut 12 due to the rolling movements of the balls 13, the lubricant must be frequently supplied into the interior portion of the ball nut 12. This raises a problem that a large amount of lubricant is necessary to control the wear of the ball screw shaft and ball nut.
The present invention aims at eliminating the drawbacks found in the conventional ball screw. Accordingly, it is an object of the present invention to provide a ball screw which is capable of reducing the supply amount of lubricant to be supplied into the interior portion of a ball nut.
In attaining the above object, according to a first aspect of the present invention, there is provided a ball screw, comprising: a ball screw shaft having a first spiral groove formed in an outer periphery thereof; a ball nut having a second spiral groove formed in an inner periphery thereof for defining a rolling passage between the first spiral groove and the second spiral groove for spherical-shaped rolling elements; and a cylinder disposed on the outer periphery of the ball screw shaft, one end of the cylinder being fixed to an end face of the ball nut, and the other end of the cylinder being closed, wherein a lubricant filling space is formed between a leading end face of the ball screw shaft and the closed other end of the cylinder.
According to the first aspect of the present invention, in case where the leading end face of the ball screw shaft is made to approach the closed end portion of the cylinder, the lubricant filled into the lubricant filling space is pushed out from the lubricant filling space, and the lubricant is allowed to flow between the ball screw shaft and cylinder, then the lubricant is supplied into the interior portion of the ball nut. Therefore, the lubricant can be supplied into the interior portion of the ball nut without supplying the lubricant from a grease nipple mounted on the ball nut. That eliminates the need to fill and store an extra amount of lubricant in the interior portion of the ball nut, thereby being able to reduce the supply amount of lubricant to be supplied into the interior portion of the ball nut. And, there is also avoided the possibility that an extra amount of lubricant can be pushed out from the interior portion of the ball nut. In addition, it is possible to prevent the periphery of the ball nut from being soiled with the lubricant pushed out from the interior portion of the ball nut.
According to the present invention, as set forth in a second aspect of the present invention, by forming a lubricant discharge opening in an end portion of the ball nut situated on the opposite side to the cylinder, the lubricant supplied into the interior portion of the ball nut can be collected from the lubricant discharge opening.
Also, according to a third aspect of the present invention, there may be disposed a pipe passage disposed on the outside of the cylinder, for returning lubricant discharged from the lubricant discharge opening to the lubricant filling space; and a check valve disposed on the pipe passage, for restricting a flow of lubricant from the lubricant filling space to the lubricant discharge opening. According to this structure, the lubricant discharged from the lubricant discharge opening can be supplied again into the interior portion of the ball nut from the lubricant filling space.
Further, according to a fourth aspect of the present invention, by disposing a lubricant filtering unit on the middle portion of the pipe passage, polluting substances such as worn powder can be removed from the lubricant.
In addition, according to a fifth aspect of the present invention, in case where there is disposed a lubricant supply unit on the cylinder, for supplying lubricant into the lubricant filling space, the lubricant can be filled into the lubricant filling space easily.